Bariatric surgery is a highly effective and increasingly popular treatment for severe obesity, but these procedures may lead to accelerated bone loss. Roux-en-Y gastric bypass (RYGB) is the most frequently performed bariatric surgery procedure in the U.S. and worldwide. Previous studies suggest that RYGB leads to significant bone loss in the initial 2 years after surgery. Longer follow-up data on the effects of RYGB on bone density and structure are currently lacking. In addition, there is little known about the skeletal effects f vertical sleeve gastrectomy (VSG), the 2nd most popular type of bariatric surgery. The goal of this application is to prospectively evaluate the comprehensive skeletal health of morbidly obese adults in the 5 years after RYGB (Aim 1), and to compare the differential effects of RYGB and VSG on the skeleton (Aim 2). This project will involve assessments of bone density, structure, and microarchitecture by dual-energy x-ray absorptiometry (DXA), quantitative computed tomography (QCT), and high-resolution peripheral quantitative computed tomography (HR-pQCT), as well as laboratory evaluation of metabolic bone parameters. This application will answer clinically important questions about the long-term safety of RYGB for bone, and will aid clinicians and patients who are weighing the risks of RYGB and VSG on skeletal outcomes. Furthermore, differential effects of RYGB and VSG might provide further insight into the pathophysiologic mechanisms underlying bone loss after bariatric surgery, which are currently unclear. Though the benefits of bariatric surgery on reducing obesity comorbidities are well established, the potential for adverse effects on skeletal integrity remains an important clinical concern that needs further study and clarification.